How the Subsurface Tropical Pacific Responds to Subtropical Surface Cooling: Implications for Cross-Equatorial Transport

Abstract Extratropical energy forcing exerts a strong influence on tropical sea surface temperature (SST) via both oceanic and atmospheric teleconnection pathways. While recent studies have emphasized the role that coupled surface ocean–atmospheric feedbacks play in communicating subtropical temperature anomalies to the tropical Pacific, details of the subsurface adjustment remain unclear. In this work, we explore how the Pacific Ocean adjusts to Northern Hemisphere temperature forcing using a novel set of ocean-only simulations in which SST is relaxed to a specified anomaly in the northeast Pacific. We clarify the time scale of the subsurface extratropical–tropical teleconnection (including the so-called ocean tunnel) by tracking the transient adjustment, and we find that cooling in the northeast Pacific dynamically adjusts subsurface temperature and circulation through baroclinic wave activity. Within ten years of forcing, the equatorial Pacific features a shoaled thermocline and a La Niña–like SST cooling in the eastern equatorial Pacific. In the quasi-steady response, forced planetary waves create horizontal temperature asymmetries which then drive a cross-equatorial shallow overturning cell response via thermal wind balance. This cell transports heat to the cooled hemisphere. We find similar temperature patterns and cross-equatorial responses when forcing is applied in the northwest and southeast Pacific. Our results provide insight into how hemispheric surface temperature forcing can drive cross-equatorial adjustments without any changes in wind. This adjustment has important implications for how the equatorial Pacific may respond to both natural and anthropogenic subtropical forcings, and it creates a more complete understanding of meridional energy transport within the coupled ocean–atmosphere system.